The present invention relates to a phase correction feedback system for adjusting the phase of an analog signal derived from a digital signal. A typical use would be in digitally based communication channel receivers or monitors. In such a monitor, for instance, such phase correction systems are especially useful in circuits for detecting channel degradation.
Communication channels can be entirely analog extending from the information source to the transmitter, and then along the channel link to the receiver, and finally to the user of the transmitted information. Communication channels can also be digitally based, either totally or partially.
A typical situation is to have a digital information source but where the information is sent in analog form over an analog channel link to a receiver which reconverts the information to a digital format. If the receiver circuits of the system are not of an ideal or optimal design, the receiver circuits may contribute to degradation in the incoming baseband analog signal. In these circumstances, the timing, or clocking, signal of the receiver, derived in the receiver from the incoming baseband analog signal, cannot be an accurate indication of the actual timing intended for events occurring in the incoming baseband analog signal. For a monitoring system used to monitor such a channel in which monitoring system these receiver circuits are duplicated, this receiver derived clocking signal, hereinafter termed the BAUD CLOCK signal, cannot then be relied upon for monitor system timing, and, further, there may be phase delays in the monitor system which must be compensated.
For these reasons two further timing, or clocking signals may be derived in such a monitor system, the MONITOR SYSTEM CLOCK and the PHASE CORRECTED BAUD CLOCK signals, which are used for monitor system timing. The PHASE CORRECTED BAUD CLOCK and MONITOR SYSTEM CLOCK signals, although obtained from the BAUD CLOCK signal, are developed in a feedback loop intended to correct some of the degradation which might occur in the BAUD CLOCK signal and to compensate monitor system phase delays.
The receiver circuit phase-lock loop which derives the BAUD CLOCK signal may be subject to temperature variation, a situation which could possibly be eliminated in the actual receiver by maintaining the environment thereabout at a relatively constant temperature level. This is a very gradually occurring source of error for most phase-lock loops which could reasonably be adjusted away in practice, and so should probably be eliminated from the causes of timing variation in the timing signal derived from the incoming signals being monitored and used to time the operation of the monitor system.
On the other hand, the phase-lock loops used in the receiver circuits are quite often affected by noise level in a rather dynamic manner and these probably should be allowed to affect the timing signals used in operating the monitor system. This is because the errors, reflected as phase jitter, etc., can cause signal degradation which cannot be adjusted away at the channel receiver. Thus, there is a need for a slow-acting, timing control feedback loop to correct certain slowly changing timing errors in the timing signal extracted from the incoming baseband analog signal. Yet, this loop should not remove the dynamic timing errors which surely should be considered for the effect they may have on the performance of the digitally based communication channel.